EE 403W, Section 4 Spring 2008

Wireless Rear Surround Sound Speakers

May 6, 2008

Dylan Gaffney dmg5015@psu.eduOwen Gaffney ojg5000@psu.eduColin Kearney csk141@psu.edu

Michael Messmer mem397@psu.edu

TABLES OF CONTENTS

PageLIST OF FIGURES AND TABLES ……………………………………………… 3

1.0 Background………………………………………………………………………... 4 2.0 Objective…………………………………………………………………………... 43.0 Project Description………………………………………………………………… 4

3.1 Requirements……………………………………………………………… 53.2 Transceiver………………………………………………………………… 53.3 ADC/DAC…………………………………………………………………. 63.4 Amplifier…………………………………………………………………… 63.5 Microcontroller…………………………………………………………….. 63.6 Loudspeaker................................................................................................... 7

4.0 Technical Approach………………………………………………………………… 74.1 Transceiver…………………………………………………………………. 84.2 ADC/DAC………………………………………………………………….. 104.3 Amplifier…………………………………………………………………… 134.4 Microcontroller…………………………………………………………….. 144.5 Loudspeaker................................................................................................... 15

5.0 Testing and Evaluation….………………………………………………………… 216.0 Problems and Solution…………………………………………………………….. 227.0 Team Members…………………………………………………………………….. 22 8.0 References………………………………………………………………………….. 23

2

LIST OF FIGURES AND TABLES

PageFigure 1: Block Diagram (Nordic Semiconductor)………………………………… 5Table 1: Requirements……………………………………………………………… 5Figure 2: PIC16C774 Pin Layout…………………………………………………… 7Figure 3: Transceiver communication………………………………………………. 8Figure 4: Schematic of Transceiver…………………………………………………. 8Figure 5: Layout of Transceiver…………………………………………………….. 9Figure 6: ADC pin layout........................................………………………………… 10Figure 7: Simple DC offset circuit…………………………………………………... 11Table 2: ADC control pin settings………………………………………………….... 11Figure 8: I2S summary…………………………………………………..................... 12Figure 9: DAC pin layout……………………………………………………............. 12Table 1: DAC control pin settings………………………………………………… ....12Figure 10a: Amplifier circuit……....................................…………………………… 13Figure 10b: Amplifier power supply…………………………………………………. 14Figure 11: SPI bus............................................………………………………………. 15Figure 12: Woofer impedance and SPL……………………………………………. ...17Figure 13: Woofer crossover.......…………………………………………………….. 18Figure 14: Woofer response........................................................................................... 18Figure 15: Tweeter crossover......................................................................................... 19Figure 16: Tweeter response.......................................................................................... 19Figure 17: Overall loudspeaker response....................................................................... 20Figure 18: Finished loudspeaker.................................................................................... 21

3

1.0 Background

This project is a wireless system that transmits high quality sound to speakers. It is designed for use in a home theater surround sound system. Instead of having to run wires to the back of the room by going through the wall or in the ceiling or over the floor, the rear speakers will be connected to the surround sound wirelessly. This system transmits sound digitally at a frequency of 2.4 GHz.

Because this is a generally new product, there are many problems with the commercial wireless speaker systems. One of the issues is how the audio is transmitted. Wireless speaker systems use infrared or radio frequency to transmit the signals. Infrared requires the line of sight to transmit the signal, so depending on the physical layout of a room infrared might not be practical. Radio frequency transmission does not need line of sight but does present other problems. If an analog signal is transmitted, it can pick up noise and interference from other wireless products which can distort the sound. When these speakers are not in use, there are sometimes crackles and pops heard through the system. Another problem with the wireless speakers on the market is the power produced. Many of these speakers do not produce more than 25 watts. Lastly the wireless systems on the market now are very expensive with prices reaching almost five hundred dollars.

This project addresses many of the problems with the present systems mentioned above. Because it transmits the audio using radio frequency, the physical layout of the room will not affect the signal. It also transmits the signal digitally which should eliminate the noise and interference that is associated with an analog signal. Transmitting at a frequency of 2.4 GHz allows a fast data transfer rate which will produce high quality sound. Lastly an amplifier that is placed on the receiver allows the speakers to deliver more power than what is offered on the market. These issues are all addressed while keeping the cost of the product lower than the market price.

2.0 Objective

The objective of this project is to design a wireless system to transmit audio to speakers in a home theater application with high quality sound. This project also is design to reduce the noise that is present in other wireless speakers while keeping the cost down. The speakers also produce power comparable to wired speakers.

3.0 Project Description

This project’s goal was to develope a wireless audio transmitter and receiver system for transmitting audio signals in a home environment and to produce an amplified signal to drive loudspeakers. The product transmits a high fidelity audio signal with low distortion across an adequate range for home theater applications. The design is based around the Nordic Semiconductor nRF24Z1, a transceiver designed for audio streaming. The chip supports both I2S and S/PDIF interfaces for audio I/O.

4

Interfacing with analog audio input and output signals is done with external A/D and D/A converters. Control of the chip is provided with a microcontroller using the SPI serial interface. Important considerations have been taken with PCB design because the project involves frequencies in the RF range.

Figure 1: Block Diagram (Nordic Semiconductor)3.1 Requirements

This project’s goal was to conform to the following table of requirements and specifications. Each requirement and specification has both a threshold value that must not be exceeded and an objective value that the project strives for.

Table 1: Requirements

Req/Spec Threshold ObjectiveTransmitting Distance >20ft 30ft

Frequency Range 20-20kHz 20-20kHzTHD <5% <1%

Channels 2 2Power >25W/channel 50W/channel

Requirements and Specifications

3.2 Transceiver

This project needed a transmitter and a receiver to transmit a signal digitally and have a fast enough bit rate. The Nordic nRG24Z1 transceiver fit this need. This transceiver is a single chip that transmits an RF signal at 2.4GHz digitally. It has a 4Mbit/sec RF link which is true lossless audio (“Nordic Semiconductor”). The Nordic also uses I2S interface which works perfect for audio support. This chip is also easy to set as either a transmitter or a receiver with the use of a mode pin being set to high or low. Lastly this is an inexpensive chip costing about six dollars.

3.3 ADC/DAC

5

The audio input for our project will be a stereo analog signal for rear surround speakers, most likely from a surround sound receiver. Analog and digital signals each have their own advantages and disadvantages in systems. One of the main disadvantages of analog signals is their susceptibility to noise and distortion. Transmitting an analog signal wirelessly would add even more distortion due to interference and problems inherent in wireless transmission of signals.

Digital signals are much less susceptible to noise when transmitted wirelessly, and are therefore able to produce much less distortion at the output. Therefore, we will need to convert the analog signal to a digital signal before transmitting it over the air. To do this, we will use an analog-to-digital converter, (ADC). The higher number of bits on the ADC will result in a higher resolution and a better representation of the input signal, but will also increase the required rate of data transmission. We will use a ADC with either 16 or 24 bits, both fairly standard in the audio industry. In order to satisfy the Nyquist Theorem, the sampling frequency will need to be at least twice the highest frequency in the input signal. We will use a sampling frequency of 44.1kHz. Listed below are our ADC requirements:

• 16 or 24 bits• 44.1kHz sampling frequency• Stereo• I2S output

The digital signal transmitted wirelessly will need to be converted back into analog before it can be sent to an amplifier and a loudspeaker. Therefore we will need a digital-to-analog (DAC) before the output of our system. The requirements will be similar to the ADC in that it needs to be stereo, have 16-bit resolution, and have a 44.1kHz sampling frequency. Instead of an I2S output, however, it will have an I2S input and an analog output.

3.4 AmplifierThe amplifier is needed to amplify the received signal to drive a loudspeaker. Because we thought that a lot of commercial surround speakers are underpowered, we wanted something with a high power output and ultra low distortion.

3.5 MicrocontrollerThe nRF24Z1 requires configuration by writing to its internal registers. Many of the register values are crucial to the operation of the chip. The internal registers are written by the use of a microcontroller. The microcontroller is programmed using a computer to go through a boot sequence each time the device receives power to make sure that the internal registers of the nRF24Z1 are set to the correct values for operation. The microcontroller communicates with the nRF24Z1 through the use of a Serial Peripheral Interface (SPI) bus. The microcontroller will act as the master device and the nRF24Z1 will serve as the slave device. This design used the PIC16C774 for the microcontroller. This PIC is a 40 pin dip package and is designed to be used as the master device on SPI interface (“Microchip”).

6

Figure 2: PIC16C774 Pin Layout

3.6 Loudspeaker

A loudspeaker is required to reproduce sound. We will build a two-way bookshelf speaker that could be used in a rear surround sound environment.

4.0 Technical Approach

In order for the system to operate probably, all of the components had to be connected in a certain order and run on the same master clock. When power is provided to the circuit, the microcontroller runs through the code and sets the register values of the transceiver. This turns on the master clock of the transceiver which is connected to the rest of the circuit. The system is now ready to transmit audio. An analog audio signal is sent into the system and first encounters the ADC. The Burr-Brown PCM1802 ADC converts the stereo audio to a 24bit I2S signal which is sent to the transceiver. The Nordic nRF24Z1 transceiver transmits the digital audio at 2.4GHz and 4Mbits/sec for true lossless audio. There is another identical transceiver set to receive the signal. This is accomplished by the mode pin of the chip. Once the audio is received it is sent to the DAC. The Burr-Brown PCM1754 DAC then converts the 16bit I2S digital output of the transceiver to left channel and right channel analog signals. These signals are sent to the class AB amplifier which provides around 100 W to the 8Ω speakers.

Along with the setup of all the components, the design and construction of each individual component is critical for the system of operate properly. An in-depth description of each is shown below.

4.1 Transceiver

The transceiver is the heart of this project. The Nordic nRF24Z1 takes the I2S signal provided by the ADC and sends it to the other transceiver at a frequency of 2.4GHz.

7

The transmitter and the receiver are able to communicate to each other with the control channel as shown in Figure 3.

Figure 3: Transceiver communication.

Nordic Semiconductor provided the schematic of the transceiver circuit shown in Figure 4. From the schematic the layout was made using Advanced Design System. This layout is shown in Figure 5.

Figure 4: Schematic of Transceiver

8

Figure 5: Layout of Transceiver

Due to the sensitive nature of RF circuitry, the transmitting and receiving circuit layouts required careful consideration. It is of the utmost importance that the components lie in as close proximity to the transceiver as possible. This is accomplished through the utilization of small surface mount components in as compact a layout as possible. Since circuit noise is detrimental to any RF application, the bottom layer of the PCB is a dedicated ground plane. All free space on the top layer of the chip is also filled by a ground plane, which is connected to the bottom layer throughout the PCB through the extensive use of vias. The heavy use of ground planes prevents noise from entering the signal.

When assembling a layout such as this for mass production, a technique known as reflow soldering is employed. This technique utilizes a stencil to cover all areas of the PCB except the component pads. Solder paste is then spread across the stencil to apply just enough to effectively connect each component. The stencil is then removed and the components are placed in their correct locations. Once this is completed, the entire PCB is placed in an oven and baked following a specific temperature curve. This melts the solder paste and forms the desired joints between the components and the PCB.

Due to the expensive equipment required to complete reflow soldering—the process is usually almost completely automated, this did not provide a feasible way to assemble the prototype model of this design. Because of the use of small surface mount components, soldering a prototype model by hand becomes inherently difficult. This process was mitigated through the use of a magnifying glass and solder paste. Small amounts of solder paste were applied to the pads followed by the parts themselves. Heat was then applied to the pads, melting the solder paste and securing the part in place. This process was then repeated at each terminal of the part until it was electrically connected to the PCB. Soldering the transceiver in place provided the most difficult challenge due to the extremely small, leadless package. It was imperative that painstaking care was taken when placing the chip on the PCB and each pad was carefully checked to ensure that none of the pins were shorted together.

4.2 ADC/DAC

After we had defined our required specifications for the ADC, we began searching for ICs that could do the job. We eventually came upon the Burr-Brown PCM1802. It has 24-bit resolution, is I2S compatible, and has an adjustable sampling frequency from 16kHz to 96kHz. Below is a pin diagram with a typical application:

9

Figure 6: ADC Pin layout

C3 and C4 are bypass capacitors and are actually 0.1uF and 10uF capacitors wired in parallel. C5 and C6 are also 0.1uF and 10uF capacitors wired in parallel. R1 is used when the analog power supply is noisy, and should be 1kΩ. The control pins need to be set to certain settings in order for the ADC to work as desired. Because the ADC only works with single ended analog inputs ranging from +1V to +4V, a 2.5V DC offset was added to the inputs to center it in the allowable region. This was simply done using the +5V analog supply and a resistor network.

Figure 7: Simple DC offset circuit

Then the internal high pass filter must be disabled in order to pass the DC offset. This can be done by setting pin 8 to high. The ADC must be run in master mode in order to generate the data clock and the L/R clock which are output to the transceiver along with the data. The table below summarizes the settings for the control pins:

10

Table 2: ADC control pin settingsPin Number Pin Name Setting Reason

7 Power Down High Normal Operation Mode8 HPF Bypass High Pass DC component to output16 Oversampling Low x64 Oversampling rate17 Format 0 High18 Format 1 Low

I2S, 24-bit

19 Mode 0 High20 Mode 1 High

Master Mode (256 fs)

Using the above settings, the transceiver can receive a valid I2S audio signal. I2S, or Inter-IC Sound, is a serial bus link developed for digital audio. It is commonly used to transmit PCM data from a CD transport to a DAC in a CD player. As can be seen in the figure below, it is made up of 3 lines: a continuous bit clock (BCK), a L/R select clock (LRCK), and a serial data output (DOUT).

Figure 8: I2S summary

We chose to use the I2S bus to transmit between the ADC and transmitter as well as the receiver and DAC because of it's common use in the audio industry and it results in a low jitter connection.

We chose to use the Burr-Brown PCM1754 DAC because it met the specifications that we needed. Below is the pin diagram with a typical application:

11

Figure 9: DAC pin layout

The circuit for the DAC is considerably simpler than the ADC circuit. The table below summarizes the control pin settings for the configuration we needed.

Table 3: DAC control pin settingsPin Number Pin Name Setting Reason

13 DEMP Low De-emphasis Filter OFF14 MUTE Low Mute OFF15 Format Low 16-bit I2S Data Format

12

AC coupling capacitors are connected on the output which removes the DC components. An external LPF at the output removes noise generated above the Nyquist frequency by the Delta-Sigma DACs. The analog output signals can then be amplified and sent to the loudspeakers.

4.3 Amplifier

Amplifier SpecificationsInput Sensitivity: 1.228VRMS to achieve full outputOutput Power: 335W continuous power into 4 ohms, 225W into 8 ohmsInput Impedance: 47k ohmsTHD: 0.001% @ 1kHz full output, 0.02% @ 20kHz full outputGain: 32.6dB

Figure 10a: Amplifier Circuit

To simplify circuit construction, we decided to go with the National Semiconductor LM3886 Audio Power Amplifier as opposed to building an entirely discrete amplifier. The LM3886 is able of supplying up to 68W of continuous power to a 4 ohm load or 38W into an 8 ohm load. It also exhibits very low THD+N, less than 0.03% over the entire audio spectrum at the rated output. The maximum supply rail

13

voltage is 84V, which if using a bipolar supply would give a maximum supply of +-42V. The maximum output current is rated at 11.5A.

Our design features four LM3886s connected in a bridge/parallel configuration. The outcome of this is a very high power amplifier solution that far exceeds the capabilities of one LM3886 alone, while maintaining reasonable power dissipation levels within each amplifier. The input buffer and the unity gain inverting amplifier are Burr Brown OPA2132s, which were selected for their low distortion and high drive capabilities. The input first goes through a coupling capacitor to block DC from the circuit. The input impedance is set by the input resistance at 47k ohms. All of the LM3886s are in an inverting configuration, so the half of the circuit that feeds the positive output is first inverted using a unity gain operational amplifier. This way the signal is inverted twice, ultimately keeping the phase shift at 0 degrees. The half that feeds the negative output is the same, just without the inverting operational amplifier at the input, resulting in a phase shift of 180 degrees. The output resistors are required to keep the amplifiers in parallel from fighting each other at the output if they have slightly different gains. The gain setting resistors should be closely matched so that each amplifier gain is as close to the others as possible. The gain on each LM3886 is set by the feedback resistor divided by the input resistor. Therefore the gain of each LM3886 is set at 21.5 V/V. Because of the bridged configuration, the voltage gain is effectively doubled to 43 V/V. With a line level input (1.228VRMS), this leads to the output being 52.8VRMS. This will require bipolar supply rails of at least +-37V to allow full output without clipping.

Our power supply contains a 500VA toroidal transformer per channel to make sure that the amplifier will receive ample power. It is an unregulated raw DC power supply that supplies +42V and -42V, enough to provide full output with a line level input. Each supply rail has over 10,000uF of filter capacitance.

Figure 10b: Amplifier Power Supply

4.4 Microcontroller

In order for the transceiver to operate properly it needs to be controlled by a microcontroller. The microcontroller sets the register values within the transceiver. When these registers are set, the masterclock of the transceiver is turned on and is

14

ready to transmit. The microcontroller and the nRF24Z1 can communicate with each other through the use of an SPI bus. SPI is a standard for serial communication which uses four wires and is often used in digital electronics to accept a serial stream of bits. A synchronous clock is used to shift serial data into and out of the microcontroller in blocks of 8 bits. The four logic signals used in SPI buses are:

• SCLK – Slave Clock• MOSI/SIMO – Master Output/Slave Input• MISO/SOMI - Master Input/Slave Output• SS – Slave Select (active low)

Figure 11: SPI bus

There are over 90 different register values which need to be set. This is done through assembly code assembled on MPLAB and programmed using WinPic. Because the output of the Microcontroller is 5V and transceiver’s input is 3.3V, a simple voltage divider is placed between the microcontroller and transceiver.

4.5 Loudspeaker

Driver Selection

The first step in designing the loudspeakers were to select drivers that would work well together. We wanted to build a simple 2-way bookshelf speaker so we needed to find a tweeter and a midbass driver that would complement each other in the crossover region. We eventually chose the Peerless 850122 and the North Creek Music D25.

Vas: 16.36 litersFs: 50.14 HzQms: 2.30Qes: 0.43Qts: 0.36

Peerless 850122 6½” Midwoofer

15

Re: 6.05 ΩSPL: 87 dB 2.83V/1m

Fs: 950 HzNominal Impedance: 6 ΩSPL: 90 dB 2.83V/1m

North Creek Music D25

Both drivers provide excellent performance for the money. They are both fairly flat in the frequency bands they will be operating in. Although the North Creek tweeter is more efficient than the Peerless woofer, this can be compensated for by simply putting a resistor in the signal path of the tweeter crossover.

Enclosure Design

Vb = Qts2.87 x 15 x VasFb = Qts-0.9 x 0.42 x Fs

Vb - the required internal volume of the enclosure.Fb - the frequency that the vent needs to be tuned to.

Our calculations:Vb = 0.362.87 x 15 x 0.58 = 0.46 ft3 ≈ 0.5 ft3

Fb = 0.36-0.9 x 0.42 x 50.14 = 52.8 Hz

The following formula is used to find the port length:

Lv - is the length of the port in inches.R - is the inside radius of the vent tubeVb – enclosure volume in cubic inches.

Our calculations for a two inch inside diameter port:

16

Lv = ((1.463 x 107 x 12) / (52.82 x 794)) – (1.463 x 1)Lv = 5.1 inches

We simplified enclosure construction by simply building a 0.5 ft3 enclosure with a 5 inch long port with inside diameter 2 inches. Cabinet walls were all ¾” MDF except for the front baffle, which was 1” MDF. Note that the internal dimensions of the cabinet below provide more than 0.5ft3. This is because they account for the volume lost when mounting the drivers and the port.

With the enclosures designed, the drivers can me mounted and their frequency response and impedance measured. Lacking the proper equipment to do this, we instead used computer software to model the effects of cabinet mounting. The plot below shows the anticipated results of SPL vs. frequency (green) and impedance vs. frequency (blue):

Figure 12: Woofer impedance and SPL

The enclosure would have a -3dB frequency of approximately 50Hz, which is sufficiently low enough for rear surround sound speakers.

Crossover Design

The next step in building the loudspeakers is crossover design. Crossovers can become quite complex, as the designer must take the frequency response, speaker impedance, and phase into consideration. The crossover’s main purpose is to split up the input signal and send the lower frequencies to the woofer and the higher frequencies to the tweeter.

We used computer software to optimize the crossover for our loudspeakers. For the Peerless midbass we used a second order Linkwitz-Riley crossover at around 2kHz. There is also a series notch filter to remove an SPL peak in the midbass region.

17

Figure 13: Woofer crossover

Applying the filter to the natural response of the driver results in the following filtered response:

Figure 14: Woofer response

For the North Creek tweeter we used a third order Linkwitz-Riley crossover also at around 2kHz. The resistor R1 in the signal path is used to attenuate the tweeter slightly so that the tweeter's response is better matched with the woofer's response.

18

Figure 15: Tweeter crossover

Figure 16: Tweeter response

Combining the frequency responses resulted in the overall system frequency response of the loudspeaker that can be shown below.

19

Figure 17: Overall loudspeaker response

The overall frequency response is pretty flat throughout the audio spectrum. Further improvements were realized, but at the expense of much more complicated filters. In the end, listening tests proved that the speakers provided a good clean, uncolored sound through most of the audio spectrum, with a good soundstage and imaging.

Finally, the speakers were painted and finished before being ready for use.

20

Figure 18: Finished loudspeaker

5.0 Testing and Evaluation

A series of tests were designed to be performed at the conclusion of the prototyping in order to properly evaluate the operation of the final product. THD is tested by inputting a sine wave to the transmitter section of the product and measuring the THD produced at the output of receiver stage of the product. Comparing this to the THD value measured at the input allows the proper evaluation of THD introduced by the system at the rated load. This test was designed to be performed over a sweep of frequencies from 20-20kHz to ensure that the product operates over the required frequency range. This test was also designed to be performed at various distances to determine the maximum transmitting distance and the THD at each of these respective distances. In order to properly determine the amplifier power, the voltage levels at the load are also measured. This ensures that the proper power per channel is obtained. The obtained data from these tests would then be compared to the requirements and specifications outlined in section 3.1 to properly evaluate the product.

Due to problems that arose when trying to program the internal registers of the transceiver, not the tests were completed.

6.0 Problems and Solution

Throughout the course of this project, a number of problems arose that impeded overall completion. While most of these problems were successfully mitigated, problems still exist within the project that prevents full functionality.

21

The actual circuit layout provided the first problem of this project. Learning to use Advanced Design System provided a significant challenge. The high learning curve associated with this program prevented the timely completion of the RF circuit layout. Difficulties associated with generating the properly formatted gerber files also delayed the production of the RF PCB.

Once the RF PCB was received from the manufacturer, soldering became the second problem encountered on this project. Once the components were all soldered to the PCB, several shorts were detected on the board. The locations of the shorts were located through the careful use of a DMM and visual inspection using a magnifying glass. These shorts were then easily corrected by re-soldering the faulty joints.

The microcontroller presented the most significant problem of the project. Because of the complexity of programming the internal registers of the transceiver, the registers were not yet correctly programmed, thus preventing the transceiver from operating correctly. Troubleshooting this is an inherently difficult task as the output of the microcontroller cannot easily be measured and as a result, the problem cannot easily be located. With another two weeks of troubleshooting, this problem would be corrected and the overall design could easily become functional. Since this is the only remaining problem associated with this design, successfully mitigating the register programming will result in a working product.

7.0 Team Members

Dylan Gaffney specializes in analog circuit design, with a primary focus on audio related topics. He has built his own speakers and power amplifiers in the past.

Owen Gaffney's experience includes high fidelity audio amplification systems and loudspeaker design.

Colin Kearney has actively been involved in hobbyist audio projects for 6 years, mostly related to the field of electric guitar. He has taken classes in microcontrollers, circuit design, and computer science—all of which will help in the completion of this project.

Michael Messmer is a senior electrical engineering student at Penn State University. The classes he has taken that are relevant to this project are the following: circuit design, microcontrollers, computer science, and power electronics.

8.0 References

Engelien-Lopes, D A, “Benefits of Total Integration of Large RF Circuits.” IEE Seminar on Telemetry & Telematics, UK, April 11th, 2005

“I2S Bus Specifications”. 5 June 1996. Philips Semiconductors. 26 February 2008. <http://www.nxp.com/acrobat_download/various/I2SBUS.pdf >.

22

Karlsen, Frank , "Guidelines to low cost wireless system design", Wireless Communications, Nordic VLSI ASA, 2005

“Nordic nRF24Z1 Product Specs”. June 2006. Nordic Semiconductors. 26 February 2008.<http://www.nordicsemi.com/files/Product/data_sheet/nRF24Z1_PPS_rev3_1.pdf >.

“Reflow Soldering”. June 2002. Altera. 26 Februrary 2008. <http://www.altera.com/literature/an/an081.pdf >.

23